This invention relates generally to magnetic and electric field sensors and more specifically to an improved apparatus for supporting such sensors in order to provide quick and simple installation and alignment thereof.
As is known in the art, magnetic sensors or magnetometers are often used to measure the magnitude of the magnetic field associated with seagoing vessels. This operation is generally referred to as magnetic ranging or silencing. More specifically, ships and submarines periodically utilize such sensors to determine the magnetic signature or profile associated with the vessel since mines are activated by certain magnetic fields. The measured magnetic field information is interpreted and appropriate action taken in order to produce magnetic "silence." More specifically, often vessels have coils (i.e. degaussing coils) coupled to the outside of the hull. The magnetization of the vessel can be changed by subjecting these coils to an electrical current. With the magnetometer measurements and knowledge of the earth's magnetic field at the location of measurement, the appropriate current to be passed through the coils can be determined.
As is also known, one type of magnetometer is the triaxial flux-gate magnetometer comprised of three sensor elements disposed in a housing, orthogonally with respect to one another. Each sensor element comprises a relatively low permeability core, an excitation coil wound on the core, and a sense winding used to detect imbalances in the core brought about by the influence of external or ambient magnetic fields. With the use of suitable electronics, a periodic magnetic field is established within the core that has uniform amplitude within the core's volume.
A measure of the strength of the external magnetic field is provided by the sense winding which produces an electrical signal proportional to the core's magnetic field amplitude imbalance in the direction parallel to the sense coil's winding axis. Additional suitable electronics converts the sense winding signal into digitized data that can be readily communicated to a remote facility, such as a shore based ranging facility.
Other types of sensors often used to determine the complete signature of a ship or other vessel are alternating magnetic field (AM) sensors and underwater electric potential (UEP) sensors. Whereas the magnetometer senses the DC magnetic field associated with the vessel, the AM sensor measures the AC magnetic field and the UEP sensor measures both the AC and the DC electric fields associated with such vessel. The AM measurements are desirable since, in conjunction with the magnetometer measurements, they provide a thorough measurement of the vessel's magnetic signature. The electrical measurements achieved with the UEP sensors are desirable since mines may be activated by certain electric fields. Like the magnetometer, AM and UEP measurements require three sensor elements orthogonally disposed relative to one another.
For proper operation, it is necessary that the above-described magnetometer, AM, and UEP sensors be oriented to magnetic north and true vertical. Generally, magnetometers, AM sensors, and UEP sensors are located at permanent installation sites, such as in waters near a Naval base. At such sites, tubes are embedded vertically in the sea floor and the sensors are disposed in such tubes. More specifically, the tubes are filled with sea water and the sensor is suspended, for example by a rigid standoff pipe attached to a cap covering the tube. With this arrangement, the orientation of the sensor to true vertical and magnetic north is facilitated. The sensor is oriented to magnetic north either by an installer rotating the tube cap accordingly or alternatively such orientation may be achieved electronically. More particularly, the orientation to magnetic north is achieved by passing slowly through the null on the east/west axis and counter-rotating the cap to the null position.
This installation arrangement may be suitable in some instances, but in others, greater control and/or accuracy of the positioning may be desired. One solution to this problem has been the use of an "internal" gimbal arrangement disposed inside the magnetometer housing in which the three sensor elements are gimballed. However, due to the electrical wiring coupling the magnetometer to the shore based ranging facility and that interconnecting the internal sensor elements, the pivoting movement of such an internal gimbal may be restricted and/or the manufacture of such structure may be complex in an effort to prevent such restriction. Moreover, this apparatus may require a prohibitive length of time for proper alignment. That is, once the tube cap is rotated in accordance with magnetic north, the gimballed sensors must re-align themselves to true vertical and the gimbal may be susceptible to overshooting its desired position. Furthermore, regarding permanent ranging facilities, use of such permanently installed sensors (i.e. installed in tubes embedded in the sea floor) may not be feasible since it may take several days to travel from the permanent installation site to a battle zone and the transit could cause undesirable changes in the magnetic signature of the vessel.